A Change of Pace: What Happened?

Here is a challenging case this morning to stretch your mind…

You are called to a 71 year old male feeling "sick". He has not felt well for the past 24 hours. He describes feelings of "general aches and pains".

He has history of heart failure, for which he has an AICD.

His vitals are stable, and he does not appear to be in much distress.

Here are two continuous rhythm strips:


A short time later, his rhythm strip looks like this:

While in this rhythm, he reports that he feels "the same".


What is the rhythm in ECG #1?

What is going on in ECG #2?

What changed in ECG #3? And Why?

Looking forward to a good discussion!


  • Brendan says:

    ecg #1- demand pacing with some pvc's
    ecg#2- 100% v-paced, but rate is nearly double what it should be. I'm guessing pt. is having a pace-maker malfunction and probably needs a pacer interagation STAT, and a possible trip to the cath lab to fix the pacer.

  • Floyd says:

    Could the first one be failure to capture? it looks like the pacer spikes are ventricular but i dont see the morphology of a ventricular pacemaker. The second looks like it has capture but the st segments look excessively discordant. Is there a 12 lead?

  • capture says:

    looks like someone has a wire loose. or a bad metabolic panel

  • VinceD says:

    I’m thinking the different morphology is the result of fusion.

    Strip #1 – The first 11 QRS complexes are the result of a normal sinus rhythm with first degree AV-block. The confusion arises in that it looks like the pacemaker is setup to sense the atria and pace the ventricle if it hasn’t heard a response in about 240ms. This is very close to the patient’s intrinsic first degree AV block, to the pacemaker spike ends up superimposed on the very early portion of what was going to be the patient’s normal QRS. The ventricle now sees signal coming from both the AV node and pacemaker, and a very slight fusion beat results. The sinus rate slows for beat #12, and the demand setting on the pacemaker tries to create an escape beat just as the sinus signal is going through the AV node, resulting in another fusion beat, but one that is wider and more negative because it has more signal from the pacemaker directing it. Beat #13 appears to be the patient’s intrinsic rhythm.

    Strip #2 – This is a hodgepodge of sinus, varying degrees of fusion, and a PVC.
    Normal sinus – 1,2,4,6,9
    Fusion – 3,5,8(but this is more pacemaker initiated that supraventricular),10
    PVC – 7

    Strip #3 – There is an increase in the sinus discharge rate, so the pacemaker must be programmed to shorten the PR-interval at higher rates, resulting in what is apparently a purely ventricular-paced rhythm. The pacemaker senses the p-wave and, now that it’s setup to deliver a pacemaker spike sooner after the atrial beat, kicks in before the sinus signal even makes it through the AV node. The complex finishes just a tiny bit later than 240ms after initiation of the p-wave, so there’s the chance that there is still a very tiny (imperceptible) amount of fusion occurring, but i suspect too much of the ventricle has been depolarized and that signal from the AV-node gets eaten up before it even makes a tiny portion of the ventricle contract, making this a purely paced rhythm (but that’s just my guess). I actually didn’t know pacemakers would change their interval settings based on the patient’s intrinsic rate, but since that really seems to be the case here, I guess they can.

  • Robert says:

    ECG 1 is a demand pacer with the pts intrinsic rhythm. What appear to be pvcs are actually paceded bts.
    ECG 2 is where the pacer took over 100%

  • doobis says:

    I’m going out on a limb and could be completely wrong . . .

    #1 looks like the SA node is firing and the AV node is conducting resulting in a relatively normal PQRST wave pattern (did note the path q waves). The V. pacer is continuing to fire but is not having affecting the ventricles because of the conduction from the AV node is conducting first or more effectively.

    #2 has a PVC that hits on the T wave (R on T). This disrupts the heart’s normal electrical system.

    #3 the V. Pacer has taken over and is causing the ventricles to conduct as it is designed.

  • Floyd says:

    I would like to change my statement to the first ekg being fusion beats and the pacemaker being ventricular and the last strip being 100% ventricular.

  • Arnel C. says:

    Strip 1 – 2nd to the last complex has a pacer spike but seems to be a “hybrid” between a narrow native complex and a paced beat. So this is a fusion beat. The rest seems narrow but preceded by a pacer spike. The pacer spikes seems not to have influenced or affected the shape or morphology of the QRS. These are pseudofusion beats. It is often interpreted as malfunctioning PM but it is not. “This occurs when local activation around the sensing electrode is delayed relative to the surface ecg. The pacing pulse is delivered at the appropriate escape interval into the absolute refractory period of almost the entire paced chamber, except for the small region around the electrode “ (from Braunwalds Textbook of Cardiology).
    Strip 2 –  Still has pseuodofusion beats and native QRS complexes.  Complex # 7 I think is PVC and not paced beat or fusion beat because it came too early for a paced beat (520 ms from the preceding complex).  After that (1000 ms or 60 beats) the PM kicked in) then was interrupted by an intrinsic beat (940 ms after) then a pseudofusion beat.
    Strip # 3 – VPaced at 85/min or every 700 ms.  I have no idea why it is counting as 140.

  • Kevin says:

    I agree with the above, the rhythm looks like it is paced but basically the native conduction is winning over the pacemaker to give fusion beats or what are entirely capture beats (similar to what can happen with VT).
    The second ECG has some native beats with ST segment depression that appear scooped . He has heart failure, given that morphology, is he on digoxin? This could be dig toxicity leading to his worsening AV block leading to his pacer firing.
    The third ECG looks like completely paced beats; however, the degree of discordance is excessive and I'd be worried about infarction. The symmetric T waves also make me a bit worried about his potassium levels. Feeling "sick" is a vague complaint but can represent a silent MI or acute kidney injury, or both. 
    I'd want a stat BMP and dig level. 

  • Nick says:

    Not too familiar with the true terminology for pacemakers, but this pts pacemaker has multiple sensing/firing sites. In the first strip, there is clearly SA node activity. The pacemaker senses this, and waits for the impulse to travel down to the AV node and to the ventricles. After the PRI reaches a certain length, the pacemaker sees that the impulse never made it to the AV node/ventricles and fires, causing our pacer spike and resulting QRS. The second strip, we see some more of these type of beats, but we also see some of the pts own intrinsic beats. The first and second beats show a P wave and resulting QRS, with no pacemaker intervention. There is a single PVC in this strip (4th from last QRS). The third from last QRS is a paced beat. This beat shows a different morphology than previously "paced" beats because there is no SA activity following this, so the pacemaker fires at a different site. This is also the case in the last strip, no SA activity, so the strip has the typical ventricular pacemaker appearance. As far as any of this having any relation to the patients complaints, I wouldnt be to worried about it. People can walk around with this type of rhythm for years. I would, however, like to see a 12-lead.

  • D'waine says:

    1) Does the patient have an AICD or a CRT device? If this is a "plain" AICD, there is likely an atrial lead, which provides sensing and atrial pacing as well as a ventricular lead which provides the ventricular pacing function as well as the defibrillator function with defibrillation electrodes located along this lead in both the SVC and the right ventricle.  However, if this device is a CRT device, it has a third lead, placed in the coronary sinus which serves to pace the left ventricle.  If that is the case, the device can then pace the atria, right ventricle, and left ventricle.  This improves cardiac output by allowing bi-atrial pacing (giving atrial kick) followed by sequential ventricular pacing, getting the heart as close to normal rhythm as possible.  This may not be relevent in this case, but it's always good to know what type of device you're working with as they can create different EKG's.
    2) As stated above, the first EKG demonstrates a sinus rhythm with first degree AV-block. There are pacer spikes noted, which indicates that the native atrial impulses are not being conducted through in the maximum time interval the device is set to allow.  This results in ventricular pacing.  
    3) The second rhythm strip basically shows more of the same, with a few ectopic beats. No big deal. 
    4) If you look at the upper, left hand corner of the third EKG strip, you will see that it says "HR 140".  What I believe is occuring is overdrive pacing by the device to correct some sort of tachyarrythmia.  Do we know if any arrythmias were noted before the third strip? Possibly there was an arrythmia that the defibrillator detected and it is now overdrive pacing to restore the patient's normal rhythm.  
    5) As for this rate being 83-ish BPM in the 3rd strip vs 65-ish in the earlier strips, it would depend on what is going on with the patient.  Most pacemakers are set to be "rate responsive" meaning that when the patient is moving (as sensed by the device) the device will increase the rate that it expects the patient's heart to be beating.  This could be caused by the device sensing the patient moving in an ambulance, so perhaps this isn't clinically significant. 

  • Strip 1 100% a pace
    Strip 2 demand pacing /c ventricular aberrancy
    Strip 3 v pace

  • Jeff says:

    Ignore first ECG, If you look at the second ECG you'll notice that some beats appear paced, and some do not.  The beats that are paced and the ones that are not appear exactly the same, you will also notice that when compared to the 3rd ECG the interval from pacer spike to QRS is shorter in #2 then # 3. 
    It appears that the pt. has 1st degree block.  The pacemaker is designed to fire if the AV node has not allowed the signal through in a rate dependant amount of time following a p wave.  I believe that the AV node is sending the electrical impulse to the ventricles after the pacemaker senses a delay but before it can get a signal to the ventricles.   Virtually the pacemaker impulse is millaseconds after the AV node sends the original impulse. 
    In in 3rd EKG you'll notice in lead III that P waves are present.  However the SA rate has increased for whatever reason.  The 1st degree block duration remains the same, but with the higher HR the Pacemaker shortens the amount of time before it fires following each p wave.  This is why in the third EKG the beats are ventrical paced beats at a rate of 85'ish.  

  • Dave says:

    This is ludicrous, I had this last week and sat down with the ER doc to figure it out, and came to no conclusions.
    With my guy the first rhythm strip wasn't as clean and had a few more ectopic beats then it converted to a ventricular paced at about 90 just like #3. I didn't get to see my strip that would have represented strip #2 here.
    If you look at strip 2 you'll see that one ectopic seemingly non pacer based prematurely fired PVC (if that was english). I can only theorize that it set off the pacer off, maybe R on T? But it doesn't seem to be that close.

  • Dave says:

    Excuse me I mistyped, not "set it off", but changed it from an atrial to a ventricular rhythm

  • Nick says:

    Can we have a conclusion? This has been up for weeks.

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